Aircraft detection system from the 1920s.

3

I’ll discuss the following antennas today. I won’t be talking

about arrays of verticals, or the new Shared Apex Loop - There

are many specialized RX antennas out there, and I can’t cover

them all today!

George VY2GF asked me to deliver this presentation on receive

antennas. I found a picture of George when he was a young

Ham….

4

This is Harold H. Beverage at his Amateur Radio Station. Possibly at the

University of Maine around 1915.

Most people think of Beverage antennas when they think of lowband

receive antennas, but as we’ll see today, there are others out there that

offer good performance, even on small city lots.

5

For a transmit antenna, we want maximum possible field

strength in a given direction (or directions) at the most

useful elevation (wave) angles. We cannot tolerate

unnecessary power loss in a transmit antenna, because any

amount of transmitting loss decreases signal-to-noise ratio

at the distant receiver. Antenna efficiency is an important

issue for transmitting. It is obvious that for a given

elevation angle and direction the highest gain antenna will

deliver the strongest signal to the target area. We really do

not care if we are being heard in other directions (areas) or

not, we are only interested in the target direction.

A receiving antenna on the other hand has a different

design priority. The goal is obtaining a signal that can be

read comfortably, which means having the minimum

possible amount of QRM and noise. The important issue

when receiving is signal-to-noise ratio (S/N). The

receiving antenna providing the best performance can and

will be different under different circumstances, even at the

same or similar locations. There is no such thing as a

universal “best low-band receiving antenna.”

An example might better illustrate the concept:

9

There are a few things we have to keep in mind:

1.) If noise is not evenly distributed (which is often the case) performance will

depend on the gain difference between desired signal direction (azimuth and

elevation) to gain in the direction of noise(s). A 20dB null on the noise

compared to gain in the desired signal direction will actually improve S/N

ratio by 20 dB, if the noise from the null direction totally dominates all other

noises.

2.) If noise arrives primarily from the same direction and angle as the desired

signal (and assuming polarization of signals and noise is the same), there will

be no S/N improvement.

3.) If noise originates in the near field of the antenna, all bets are off. Anything

can happen.

Antennas with a nominal gain of as low as –15 dBi will normally not

require a preamplifier, unless you live in a very quiet rural area and

have very long, lossy feed lines. That means that Beverages, even with

feed lines of many hundreds of meters long, will—as a rule—not require

a preamplifier. Unless of course you like to see you’re S-meter dancing

up and down like a yoyo. But signal readability has nothing to see with

dancing S-meter needles. It is only a question of signal-to-noise ratio.

But we have also seen receiving antennas such as loops with gains of –

30 dBi, and that is pretty low and does require some signal boosting.

How do you know if you need a preamplifier? The rule is simple. During

the quietest moment of the day (usually at noon on 160 meters), with

your receiver set at the narrowest bandwidth you normally use, can you

hear the noise go up significantly when you switch from a dummy load

to your receiving antenna? If you can, then you have enough gain in

your system.

If you hear the receiver’s internal noise and not band noise, then you

need a preamplifier. You have a problem because:

The antenna output might be extremely low (less than –15 dBi, for

example).

The feed line might be very long and/or lossy.

You need to compensate for filter losses, splitter losses etc.

As a rule you should keep the signal level as low as possible to prevent

chances of intermodulation and overload. This is also why our receivers

have a front-end-attenuator, as well as a switchable preamplifier.

MMICs have poor IMD performance and are also very poor with even-order harmonic distortion. They have a fixed gain, which is usually way too high anyhow. Last but not least, they withstand little abuse.

12

Larry, W7IUV, has published a 160-meter preamplifier design with excellent

characteristics. Go to his Web site (w7iuv.com) for further details on this very

popular amplifier. The design of this popular preamp changes as he discovers

ways to improve it. Please check his website for the latest information.

Sergio, IK4AUY, described an excellent preamplifier as part of his article “A

High Level Accessory Front End for the HF Amateur Bands” in Apr/March

2003 QEX . The schematic of his preamplifier is shown in Fig 7-157. The

performance data are just short of spectacular:

3rd order intercept (IP3): +43 dBm

1 dB compression: +24.5 dBm

Gain: 12 dB (1 to 30 MHz)

Noise figure: 3.9 dB at 30 MHz

14

15

While often attributed to charged particles

(such as water droplets) hitting an antenna,

most precipitation static is actually caused

by intense electric field gradients in the area

surrounding the antenna. Such conditions

commonly appear during inclement weather,

when movement of particles or moisture

causes concentrated areas of charges. The

strong electric fields are responsible for

noise-producing corona discharges. The

noise comes from low-current corona

discharges from sharp or protruding objects.

(St. Elmo’s Fire)

Using an antenna at a lower height reduces

corona current—The electric-field gradient is

smaller close to the wide smooth surface of

the earth. Vertical antennas are particularly

sensitive to precipitation static; they have

pointed ends protruding upwards towards

the oppositely charged sky.

Beverages on the other hand, being near

earth, will have fewer corona discharge

problems. They also have low surge

impedances. This means the low-current

high-voltage arcs transfer very little noise

power into the antenna. Beverages are thus

How many of you have built loop antennas for receive?

17

Unlike a large loop antenna (greater than 0.5 wavelength), a Receive Loop is

much smaller (most are less than 0.1 wavelength), and exhibits a much

different antenna pattern. RX Loops deliver strongest signals in the direction

of the plane of the loop, and sharp nulls broadside to that plane.

Signals broadside to the loop induce the same voltage throughout the wire of

the loop. Because all sections of the wire are at the same potential, no current

can flow. Signals off the ends of the loop induce different voltages in the loop

wire as they travel through it, allowing a current to flow.

The advantage of the RX Loop is the sharp null. Careful positioning can block

out an interfering signal or noise, and permit the desired signal to be heard.

18

A typical homebrew RX Loop. Note the use of a separate loop inside the main

winding. It presents a better impedance match to the receiver. It also prevents

the main loop from being too heavily loaded by the receiver. This would

destroy the Q of the circuit, allowing strong, off-frequency signals to enter the

receiver’s front end.

19

The presence of metal objects near a loop antenna can upset the balance of the

system, causing the null to “fill in”. This sort of distortion can eliminate the

advantage of an RX loop – the sharp null that can be used to block an

interfering signal or noise.

20

This can be eliminated by shielding the loop as shown. This ensures that the

loop remains balanced with respect to ground despite the presence of nearby

metal objects. It has nothing to do with receiving the magnetic component of

an EM wave – this is a common misconception! Check out the website of

W8JI (www.w8ji.com) for an in-depth explanation. In reality, the shield

becomes the actual antenna, not the wire inside the loop. There are many good

and bad ways to feed and shield an RX Loop. Points to remember:

•The shield is the actual antenna

•The shield must be perfectly symmetrical away from the inner wire exit point

•The gap in the shield must be exactly opposite the grounded point

•The ground must be at the inner wire exit point

•The shield will not make an unshielded loop that is properly balanced any

quieter

•The shield only is a tool to help you balance the system IF the shield is

properly implemented

21

RX Loops mounted in the shack can be tuned manually with variable

capacitors. Tuning remote loops can be more problematic. I have tried slow

speed electric motors to turn the variable capacitor. The electric noise from

the motor makes it difficult to peak the antenna. Varactor Diodes, which

exhibit a changing capacitance as the voltage is varied, can be used instead.

The voltage is fed up the coax, using a DC blocking capacitor to protect the

receiver.

22

You can use a feedback loop to increase the gain and the Q of the loop. This

circuit is essentially an Armstrong oscillator that doesn’t quite oscillate. The

signal provided by the feedback loop must be adjusted until the circuit is on

the verge of oscillating. At that point, the gain increases and the bandwidth

narrows. It takes some skill to use this type of RX Loop, but it is apparently

an excellent accessory. Some sort of potentiometer might be necessary to

better control the amount of feedback.

You can use a magnetic loop with a broadband current amplifier in lieu of the

usual resonating components. Jeff VE1ZAC built the loop seen here. His

antenna utilizes an aluminium 1 meter diameter loop with dual “Hoops” to

make it look fatter and attain a lower self-inductance. It measures at 1.48 uHy

which is apparently quite good for a loop like this. It is mounted on a 2 M

mast and sits on a TV rotator allowing remote control from inside the shack. It

uses two aluminum flat bars 3.1 meters long, 1 1/4″ wide and 1/8″ thick. 1 m

diam. L= 1.45 uH

inductance is critical. The ideal loop is fat with very low inductance. Fatter is

better, but it is not strictly necessary to have a full metal loop to get the

improved low inductance result. A second loop place parallel to the first and

offset by about 8” produces a lower inductance through mutual coupling.

The amplifier is built by LZ1AQ at Active Antennas. It actually has three

receive modes built in, all controlled from the remote shack end switches. You

can have one loop, a second crossed loop or other combinations. Plus, the built

in switching allows using your two loops, or one loop and a short wire as a

small dipole connected to a voltage amplifier. In Jeff’s case he used the loop

mast for the second element. It works very well. The entire system is well

made and arrived complete and intact in a timely fashion via airmail. The units

are checked out and adjusted before leaving Bulgaria, and there is excellent

documentation showing how to hookup and test the system. All connectors and

hardware are provided. You are responsible for the loop, some shielded CAT5

cable to hook the unit up, and a power supply. 23

25

Because of the poor sensitivity compared to many other RX antennas,

conventional RX Loops are probably better suited for SWL, Broadcast Band

and LF Beacon DX’ing, where signals are going to be stronger than on the

Amateur bands. The newer amplified broadband loops seem to offer better

performance, and may be worth checking out.

27

These elongated terminated loops are really a simple pair of verticals

with a horizontal feed line, one being basefed, the other being fed via

the top wire. That provides crossfire phasing. When the element

spacing is less than λ/4, this always fires towards the feed point end of

the array. When properly terminated the loops show nearly constant

current at all points on the loop. This constant current is achieved by

terminating the far end of the loop in the same impedance as the feed

point. Thus, the portions considered to be the vertical “elements” have

equal current, with a phase equal to 180° plus the electrical length of

the connecting wires, which is slightly more than the element spacing.

In other words, the terminated loop is like two constant current verticals

with classic end-fire phasing. The only difference is the horizontal

component in the radiation due to the radiating delay line (sloping or

horizontal) of the terminated loops. This is the operating principle for the

entire family of elongated terminated loops. Sloping (inclined) wires

used in some of the designs are just like a horizontal wire in

combination with a vertical wire, and the same reasoning as above can

be used.

28

Floyd Koontz, WA2WVL, is the originator of the EWE. His QST articles are must-reads

on this novel receiving antenna. Incidentally, the curious name “EWE” came about

because Floyd noted that his new antenna looked very much like an upside-down

letter “U.” He jokingly submitted a drawing of a female sheep—a ewe—to headline his

February 1995 QST article, which he impishly titled with the triple entendre “Is This

EWE for You?”

A EWE is small, requires little engineering, and can be designed to cover 80 and 160

meters. Moreover, the EWE is low profile and can be built for little money. In

appearance, the EWE resembles a very short Beverage, though in fact it as an array

of two short vertical antennas.

The horizontal wire is only about 5 to 15 meters long and is about 3 to 6 meters above

ground. In other words, an EWE can fit in many tiny yards. The original description by

WA2WVL uses 3-meter high “elements” with a spacing going from 4.5 to 18 meters,

depending on the band (80 or 160 meters).

A good rule of thumb is to build an EWE where the length is twice the height. This rule

can be followed for elements going from 3 to 6 meters in height, and will result in an

optimum termination resistance of approximately 1000 Ω for both bands. The smaller

the array, the easier it is to obtain a good deep null, but the lower the output level. The

larger versions will be near the cut-off frequency, where a deep null cannot be

obtained.

The output (gain) of the antenna depends on its size. The gain for 160 meters varies

typically between –20 dBi and –30 dBi over average ground, and is about 9 dB higher

on 80 meters. The inverted-V shaped EWEs looks quite attractive and can easily be

made into a “hand-rotatable” receiving antenna: Just walk out in the garden and

anchor the sloping wires to a different set of ground rods. It should be useful for

DXpeditions as well.

The value of the terminating resistor is quite critical if you want to obtain a good notch

in the back response. Since the ground resistance is part of the terminating resistance,

a good low-impedance, stable ground is required. The ground resistance of a single

ground rod may vary from as low as 50 Ω to as much as several hundred Ω. Although

29

Ewe Antenna at KC4HW

It has been reported that the best way to tune an array for best F/B is to

find a medium-wave BC transmitter in the back of the antenna. Using a

medium-wave signal during day time ensures that you will receive it on

ground-wave, and as such have a constant and stable signal source.

Tune the terminating resistor for a full notch, striving for a minimum of

25 dB. This value will be very close to the best value for 160 meters and

close to what’s best on 80 meters.

You should use broadcast stations between 1600 and 1700 kHz to do

the null adjustment if you can. It is impossible to reliably adjust the

termination resistance using signals arriving by skywave. You could also

use a nearby local ham (in-line off the back) or set up a small signal

generator off the back, using a small vertically polarized antenna, at a

distance of at least 1 λ from the EWE (which means outside its near

field).

30

A major drawback to the EWE is its extreme sensitivity to any change in

the local soil conductivity. EA3VY was the first to come up with the idea

of adding a ground wire between the bottom of the two verticals as an

effective way to minimize the effect of different soil conductivities.

Many dimensions will work but only the optimized ones provide the best

directivity over the largest frequency range with a single termination

resistor. These standard values developed by EA3VY and K6SE are

4.27 meter (14 feet) high by 8.84 meters (29 feet) wide, with a bottom

wire height of 2 meters (6 feet) above ground. These dimensions result

in an antenna that has excellent characteristics from 7.5 MHz down.

This configuration has a gain of approximately −28 dBi on 160 meters,

−18 dBi on 80 meters and –10 dBi on 40 meters. The loop is terminated

with 950 Ω and fed in the centers of opposite vertical sections. Its input

impedance on 160 meters is 950 Ω with a small reactive component (10

to 50 Ω).

A properly designed Flag exhibits a high F/B ratio over any type of soil

and at virtually any height above ground without need to change the

dimensions or termination value.

K6SE, who has thoroughly tested all shapes and sizes of elongated

terminated loop antennas, reports that the Flag antenna is probably the

best of all configurations from the standpoint of being broadbanded and

having gain. It has about 6 dB more gain than an equal-sized Pennant.

31

The shape and the size of this loop makes it attractive for a rotatable

version. W7IUV describes such a design on his website

(www.qsl.net/w7iuv/).

32

While optimizing the flag, K6SE and EA3VY developed the triangular-

shaped loop. He named it the Pennant because its triangular shape

resembles a flag pennant. The loop is fed in the center of the vertical

section, while the load is situated where the sloping wires meet.

The correct termination resistor for the Pennant is about 900 Ω, which

can be placed either at the point of the Pennant or in the center of the

vertical section—the results are identical. The feed point is at the

opposite end. Gain on 160 meters is about –34 dBi (which is 6 dB less

than the Flag), −24 dBi on 80 meters and –16 dBi on 40 meters. The

exact values depend on local ground conductivity. Note that the 6-dB

lower signal level is proportional to the difference in area enclosed by

the loop, which is half as much as the Flag: 1/2 voltage = 1/4 power (−6

dB).

Evaluation of the Pennant by Jeff VE1ZAC:

“This is probably the simplest, cheapest, least complex and best performing

receive antenna out there. It has a 30 foot footprint and only needs 15 to 20

feet of height to work. And work it does. It does interact with its own feed line

and with other antennas, but if you are careful with these things, it is a stellar

performer. It is not unreasonable to get 30 dB F/B with one of these, on 160 M.

A perfect antenna to aim into the SW or NE for 160M contests. Best of all

worlds ?.. have two of them, one for each direction. This antenna plays well

with fishing pole and short tree supports. Nearly every low band ham

aficionado on a small lot can setup one of these. It also works well over crappy

grounds.. like my QTH and much of Halifax. It looks like a triangle on its side.

I used this in several 160M contests with good results. Best bang for the buck

receive antenna. It is very simple to build and erect.”

33

34

A EWE in the shape of a Delta loop has

every bit as clean a pattern as the more

classic ones. I think this antenna is really

attractive for DXpeditions and for semi-

rotatable setups. Just moving the base line

around is all you have to do to change

directions. During modeling it appeared that

the best F/B was obtained with the

terminating resistor mounted approximately

20% from the bottom corner of the Delta

loop. The antenna is fed in the opposite

bottom corner.

The FOØAAA Clipperton Island DXpedition

used this antenna in their operation. It is the

only design that requires only one support

and can be easily rotated manually, a very

desirable feature for DXpedition use. The

Clipperton team found the antenna to be

very successful and many other subsequent

DXpeditions have also used the Delta

configuration.

The optimum termination resistance for this

design is also 950 Ω. The antenna was

optimized for 160 meters and its output is

about –33 dBi on 160 meters, −22 dBi on 80

and –13 dBi on 40 meters.

35

Gary Breed, K9AY, described this loop very

well in his Sep 1997 QST article (Ref 1265).

This is another variant of the EWE, where

the bottom wire of the loop is grounded in

the center. See Fig 7-128. He pointed out in

his article that the loop can really be any

shape. The diamond shape K9AY used was

dictated by practical construction

considerations rather than anything else. All

of the K9AY loops can be used on both 160

and 80 meters, although optimal

performance may require slight adjustment

of the terminating resistance, as K9AY

pointed out in his article.

While in all the previously described loops

the feed point and the termination are

separated by distance (they are at opposite

ends of the elongated loops), in the K9AY

loop “separation” is achieved by grounding

the loop between the adjacent feed and

termination points. Having the feed and

termination so close to one another makes it

easy to switch directions from a single box.

36

Using the proper relay switching setup, it is possible to insert the matching

transformer and termination relay as necessary so that 4 different directions

can be selected.

41

K9AY offered by AY Technologies.

42

K9AY has a much smaller footprint than 4 Ewes.

George Wallner AA7JV’s “Double Half Delta Loop” antenna is the latest of

the terminated loop designs. It requires two support poles, separated by

approximately 22 meters, as shown in the diagram.

The bottom wire is at approximately 1.5 meters above ground, and the higher

this wire, the less noise it will pick up. As the antenna is ground independent,

you can just move the whole thing up a few meters without changing anything

else.

43

It is important to remark that this “double” loop has a much narrower –3 dB

forward lobe angle (~100°) than the earlier described elongated loops (~140°).

Hence the significantly better RDF figure as well.

Note that there is nothing special about the dimensions or exact shape of this

antenna. The more area the wires enclose, the larger the signals (higher gain),

but the RDF may get slightly worse. Also, the larger antenna will not work on

the higher bands. (The upper cut-off is approximately where the total wire

length

reaches 1⁄4 wavelength.)

Note that thermal noise will eventually overtake signals as the antenna is made

smaller, so it is not possible to build a miniature antenna and use a preamp to

bring the signal levels up.

44

This version of the Double Half Delta Loop is used by BCB DX’ers. It is

larger to offer better performance on the AM broadcast band. These

dimensions would be a good starting point for use on the new 630m band.

45

46

The essential characteristics of a good

transformer for a loop antenna are:

Lowest possible capacitance coupling between primary and secondary

windings.

Low loss because signals are very low-level.

Good SWR is essential if you want to phase such antennas (you want

the phase angle to be the determined by the cable length!)

47

Winding Data for Binocular Transformer

Transformation Low-Z High-Z

500 Ω to 75 Ω 2 passes (1 turn) 5 passes




48

Summing Up, Feeding Elongated

Receiving Loops

- Make sure your loop is as physically balanced as possible.

- It’s better to have the loop 5 meters high than 2 meters.

- Use a transformer with separate primary and secondary windings.

- Use a transformer with the lowest possible capacitive coupling

between primary and secondary windings.

- Do not use a metal box to house the transformer.

- Run the feed line along the symmetry axis of the loop for several

meters.

- Drop the coax down to ground, where you ground the shield to a good

ground system. Insert a common-mode

choke at that point (between the ground rod

and the shack).

The dual active delta flag array is a dual delta flag array which is activated by

high performance FET followers. The FET followers basically transform the

open source voltages of the flag elements directly to 50 ohm outputs. No

antenna transformers are used, so there is no reduction of signal levels. This is

equivalent to attaching 15 dB gain very low noise figure amplifiers directly to

the flag elements. In other words, low signal level output flag antennas and

flag arrays are converted to high signal level output flag antennas and flag

arrays.

Developed by Dallas Lankford, a MW Dxer. He had done a lot of research

into receive antennas – check out his articles at the link.

50

The dual active delta flag array is a dual delta flag array which is activated by high performance FET followers. The FET followers basically transform the open source voltages of the flag elements directly to 50 ohm outputs. No antenna transformers are used, so there is no reduction of signal levels. This is equivalent to attaching 15 dB gain very low noise figure amplifiers directly to the flag elements. In other words, low signal level output flag antennas and flag arrays are converted to high signal level output flag antennas and flag arrays.

The high performance FET follower is in the box labeled PPL (which is an abbreviation for “preamp per loop”). It is not really a preamp, and the antenna element is not really a loop, but never mind. Doug NX4D coined the term PPL, and Dallas uses the term now..

Grounding the flag elements converts them into EWE elements with 6 dB additional signal output. Single EWE elements generally have poor patterns which are ground dependent. But dual EWE arrays have much better patterns and are more or less ground independent, just like dual flag arrays. Thus before sunset when greater gain is needed, the array can be operated as a dual EWE array, while near and after sunset the array can be changed to a flag array with superior splatter reduction. The reverse is done at sunrise.

51

The half size active dual flag array described in the graphic above requires

only 100' (30.5 meters) of linear space, and so it is an excellent choice for MW

DXers with limited space for antennas. Although small compared to previous

high performance MW antenna arrays, it is a state of the art MW receiving

antenna array, primarily because it uses true active delta flag antenna elements.

According to Jeff VE1ZAC: “I can confirm, they work as advertised, or better,

are easy to build, and simple to put up. I happen to have one line to the NE that

I can just fit the 100 feet needed for this short version of the antenna, and it is

a crowd pleaser( well.. me). It works on 160 M quite well too, but Dallas tells

me it would be better on 160 if it was cut for 160. I may try that next. It is

really fun to pick out all the Eu MW stations you can hear most evenings.. I

can easily detect or hear maybe 40 or 45 any particular night. Very cool. I am

looking forward to trying on 160M DX this winter. As a happy coincidence,

the plasma TV noise source is in the null of this antenna to the SW of me. I

can’t hear it at all. On bad side, because of that TV source, there isn’t much

point in reversing this thing as that plasma TV is going to dominate the band to

the SW.”

52

53

Dr. Harold Henry "Bev" Beverage (14 Oct 1893 in North Haven, ME - 27 Jan 1993) is

perhaps most widely known today for his invention and development of the wave antenna,

which came to be known as the Beverage antenna and which for the last few decades has seen

a resurgence in use within the amateur radio and broadcast DXing hobbyist communities. Less

widely known (outside of the community of science history researchers) is that Bev was a

pioneer of radio engineering and his engineering research paralleled the development of radio

transmission technology throughout his professional career with significant contributions not

only in the field of radio frequency antennas but also radio frequency propagation and systems

engineering.

[edit] Biography

Harold Henry Beverage received a B.S. in Electrical Engineering from the University of

Maine in 1915, and went to work for General Electric Company the following year as a radio-

laboratory assistant to Dr. Ernst Alexanderson. In 1920, he was placed in charge of developing

receivers for communications at the Radio Corporation of America in Riverhead, New York.

Three years later, at the age of 30, he received the IRE Morris N. Liebmann Memorial Prize

"for his work on directional antennas."

RCA named Beverage chief research engineer of communications in 1929, a position he held

until 1940. At that time, he was promoted to vice president in charge of research and

development at RCA Communications Inc., a subsidiary of the Radio Corporation of America.

He retired in 1958 from that position and as director of radio research, but continued to work

in communications as a consultant.

In 1938, the Radio Club of America presented him with its Armstrong Medal for his work in

the development of . The Beverage antenna, the citation said, was "the precursor of wave

antennas of all types." Beverage was awarded the IRE Medal of Honor in 1945, "In

recognition of his achievements in radio research and invention, of his practical applications of

engineering developments that greatly extended and increased the efficiency of domestic and

world-wide radio communications and of his devotion to the affairs of the Institute of Radio

Engineers."[1] In awarding him its Lamme Medal in 1956 the American Institute of Electrical

Engineers cited him "for his pioneering and outstanding engineering achievements in the

conception and application of principles basic to progress in national and worldwide radio

communications."

54

Harold Beverage discovered in 1920 that an otherwise nearly bidirectional

long wire antenna becomes unidirectional by placing it close to the lossy earth

and by terminating one end of the wire with a non-inductive resistor with a

resistance approximately matched to the surge impedance of the antenna. This

was the fundamental discovery in his 1921 patent.

http://en.wikipedia.org/wiki/Bidirectional

http://en.wikipedia.org/wiki/Surge_impedance

55

The Beverage Antenna relies on "wave tilt" for its directive properties. At low

and medium frequencies, a vertically polarised radio frequency

electromagnetic wave travelling close to the surface of the earth with finite

ground conductivity sustains a loss that produces an electric field component

parallel to the earth's surface. If a wire is placed close to the earth and

approximately at a right angle to the wave front, the incident wave generates

RF currents travelling along the wire, propagating from the near end of the

wire to the far end of the wire. The RF currents travelling along the wire add in

phase and amplitude throughout the length of the wire, producing maximum

signal strength at the far end of the antenna where a receiver is typically

connected. RF signals arriving from the receiver-end of the wire also increase

in strength as they travel to end of the antenna terminated in a resistor, where

most of the energy propagating in that direction is absorbed.

Radio waves propagate by the ionosphere at medium or high frequencies (MF

or HF) typically arrive at the earth's surface with wave tilts of approximately 5

to 45 degrees. Ionospheric wave tilt allows the directivity inducing mechanism

described above to produce excellent directivity in Beverage antennas

operated at MF or HF.

While Beverage antennas have excellent directivity, because they are close to

lossy earth they do not produce absolute gain (typically -20 to -10 dBi). This is

rarely a problem, because the antenna is used at frequencies where there are

high levels of atmospheric radio noise. The antenna has very low radiation

resistance (less than one ohm) and will rarely be utilised for transmitting. The

Beverage antenna is a popular receiving antenna because it offers excellent

directivity over a broad bandwidth, albeit with relatively large size.

57

Note how the pattern sharpens, and gain increases as the length is increased.

The lesson would seem to be that the longer the better for Beverage antennas.

This is not true in actual practice however!

58

One limiting factor is the phase difference that will occur

between the EM wave in free space, and the slower induced EM

wave in the antenna. As the antenna is increased in length, that

phase difference grows until the EM wave in free space is

actually subtracting from the induced wave in the antenna. Of

course, the slower the wave in the antenna (ie: the lower the

velocity factor), the shorter the antenna will be before that point

is reached.

The velocity factor of a Beverage will vary typically from about

90% on 160 meters to 95% on 40 meters. These figures are for a

height of 3.0 to 3.5 meters. At a 1-meter height the velocity factor

can be significantly lower, depending on ground quality. BOGs

(Beverages on ground) may have a velocity factor around 60%,

which means that very short, very low Beverages can exhibit

radiation patterns similar to higher Beverages that are much

longer.

This equation is used to calculate a maximum effective length

based on the velocity factor. If we use a VF of 90% for a 160M

Beverage (assuming 3 meters above the ground), then the MEL is

360 meters, or 2.25 Lamda. For a wire laying on the ground with

a VF of 60%, the MEL is 60 meters, or 0.375 Lamda.

Even before this MEL limit is reached in antennas with a high VF

however, there is another limiting factor. There is such thing as

space diversity, which means that wave characteristics change

with place. As long as you stay within a radius of approximately

two wavelengths, this usually does not cause any problems. This

is the reason why very large arrays and very long Beverages, may

actually behave differently from what the model tells us. “Longer

is better” does not hold true for Beverages (as for any large

receiving array). Besides, we have already seen that gain is not

an important factor in RX antennas – directivity is.

59

The general rule is as follows:

- Higher Beverages produce higher output

- Higher Beverages have larger side-lobes

- Higher Beverages have a higher elevation angle

- Higher Beverages have a wider 3-dB forward lobe

The height is not all that critical. Below 2

meters, Beverages can be a hazard for men

and animals. If you must cross a driveway or

small street, you can put your Beverage up

to 6 meters high and still have a working

Beverage. You can also slope the Beverage

gently up from 2 meters to 6 meters to cross

the obstacle without much harm at all. Tom,

W8JI, writes on his Web page

(www.w8ji.com): “I’ve found very little

performance difference with height, unless

the Beverage is more than 0.05 λ high.”

If the Beverage can be constructed on

terrain that is inaccessible to people, deer

and other animals, then you can consider

using Beverages at a height of 0.5 or 1

meters for added high-angle discrimination

and reduced omnidirectional pick up.

60

- The better the ground, the lower the output

from the antenna (around 6 to 8 dB

difference between very poor ground and

very good ground). But even over very good

ground Beverages have more than enough

gain.

- The peak elevation angle changes only

slightly with ground quality. For example, a

300-meter long Beverage peaks at 27° over

Very Poor Ground. The response at 10°

elevation is down 3.4 dB from the peak.

Over Very Good ground, the lobe peaks at

29°, and the response at a 10° elevation

angle is down 2.8 dB from the peak

response.

- The poorer the ground quality, the less

pronounced the nulls will be between the

different lobes. This is similar to what we

notice with horizontally polarized antennas

over real ground.

- The directivity factors (DMF and RDF) of a

Beverage antenna remains almost constant

for grounds ranging from Very Good to Very

Poor.

61

The elevation angle: The radiation angle only changes marginally (a few

degrees) between very poor ground and very good ground. The

elevation angles shown in Fig 7-67 are for Average Ground. Note the

large difference in elevation angle between long and short Beverages:

17° for a 3-λ long antenna and approximately 40° for a 1-λ long

antenna!

Gain: Fig 7-68 shows gain curves vs length, over very poor ground,

average ground and very good ground. As indicated above, the gain

figures may be a little optimistic, a known flaw with NEC-2 for antennas

close to the ground. From these curves it may seem that maximum

usable length is determined by the way gain diminishes beyond a

certain length. This is not important because gain is not an important

parameter with receiving antennas.

62

- Insulated wire may be better if antenna can rub against tree branches.

Insulation can also hide a broken conductor.

-Long spans may be necessary, so pre-stretch soft-drawn copper wire.

-Multi-conductor wire fine – solder ends together and use all conductors.

63

The theoretical impedance values calculated

using this formula assume perfect ground.

They are useful for estimating the

terminating resistor for a single-wire

Beverage and for designing matching

transformers and networks. Note that the

impedance does not change drastically with

height or wire size.

Over real ground however, the surge

impedance appears to be higher than over

perfect ground. This is because the electrical

ground is not the surface of the ground, but

a little deeper. The following correction

figures can be used:

Good ground: +12%

Average ground: +20%

Very poor ground: +30%

Typical values range from 300 Ohms for a

Bev laid on the ground, to 650 Ohms for

very thin wire at 5 meters height. Practical

values are usually 450 to 500 Ohms.

64

Riki, 4X4NJ, described such a homemade

spark gap: “The spark gap consists of heavy

solid wires—about 10 AWG, soldered to

teardrop terminals that are placed under the

screws going to the antenna and ground.

The ends of the wires are cut with “side

cutters” leaving a nice “knife edge,” and I

position the two edges very close to each

other. A piece of paper makes a nice “feeler

gauge” for this purpose. It is very effective,

most simple, and negligible cost.”

Photo is of K9DX Beverage termination.

Using fuse clips allows resistors to be

replaced easily if burnt out by nearby

lightning strike.

66

Methods to terminate a Beverage antenna.

Different terminating systems for

Beverage antennas. The version at A

suffers from stray pickup because of the

vertical down lead. At C, two in-line

quarter-wave lines terminate the

Beverage, by which the radiation from

these lines is effectively canceled. This is

not a very practical solution because of

its area requirements. This configuration

is used through the book for modeling

Beverage antennas, however. At D, the

method most widely used with real-world

Beverages. Using a long sloping section

is thought by many to reduce

omnidirectional pickup.

In reality, vertical drop is vertical drop, no

matter if it is over 20 meters, or straight

down. If it weren’t, then the K9AY and

similar antennas would not work!

67

If the terrain is irregular, minor ups and

downs in height or dips or valleys will not

ruin your Beverage performance though. If

your terrain is not flat, follow the contour of

gradual slopes and go straight across

ditches or narrow ravines without following

the contour.

69

PVC pipe with slot sawn in the top.

70

Recommended by W8JI

Try to have the wires able to slip through insulators. Easier to detect a wire

break, and lessens the chance of the wire actually breaking.

71

Recommended by W8JI

72

Not recommended by W8JI (though I use the same type of standoff insulator!)

73

Parallel Beverages

Beverages are non resonant and exhibit

very little mutual coupling. The rule of thumb

is to keep two parallel Beverages separated

by a distance equal to their height above

ground. This separation guarantees a

decoupling of at least 30 dB.

Crossing Beverages

Beverage antennas for different directions

may cross each other if the wires are

separated by at least 10 cm (4 inches).

Beverage antennas should also not be run in

close proximity to parallel conductors such

as fences, telephone lines, power lines

(even if quiet) and the like.

A field expedient way to separate two crossing Beverage antennas when I

discovered that the top wire had sagged down onto the lower wire. There was

4 feet of snow on the ground when I discovered this, so I wasn’t about to put

up another pole to hold them apart!

76

ON4UN has a discussion on the various matching transformers that could be

employed. I prefer the binocular cores recommended by W8JI. Separate

windings help prevent spurious signal pickup on the coax shield.

78

- Even with inexpensive TV quality 75 Ohm cable, losses in runs of several

hundred feet are acceptable.

-Do not suspend coax – keep on ground to prevent signal pickup on shield.

-Use flooded cable if critters a problem.

-Use RF chokes to keep signals off shield - Five turns of miniature coax

through a FT150A-F core (μ = 3000 and AL = 5000) achieves an

impedance of 1500 Ω (7 turns = 3 kΩ on 160 meters).

80

Make sure you always switch both the inner

conductor and the shield of these feed lines.

Do not connect the shields of all these feed

lines together or to a common ground. If you

want to ground them, do so at a distance of

5 meters from the switch box, to individual

ground stakes that are well separated.

If at all possible I suggest using widely

separated end posts for the Beverages, with

at least 5 meters separation and separate

grounds (Fig 7-92E). Avoid using the

antenna ground of one antenna as the

termination ground of another one that might

be running in the opposite direction.

Little or no coupling occurs between the

antenna wires, unless they are parallel and

quite close together. I have not noticed any

problem with antennas arriving at one point

under a 30° angle.

81

At A, open-wire two-direction Beverage

antenna. This uses reflecting

transformers to obtain both reflections

and proper impedance matching (see text

for details). At B, an autotransformer is

used for the same purposes.

84

When the Beverage antenna is not

terminated, the directivity will be essentially

bi-directional. In real life the unterminated

Beverage is not a true bi-directional

antenna. The SWR and the loss on the

antenna make it have some F/B. F/B is

related to standing waves on the antenna;

that is, to the reflection making it back to the

receiver. There is slight attenuation from the

back direction because of the extra loss of

the reflected wave in the wire.

A method of switching the Beverage from

unidirectional to bi-directional is shown here.

A relay at the far end of the antenna wire is

powered through the antenna wire and the

earth return via an RF choke and blocking

capacitors.

85

Can you really improve Beverages by

phasing them? Theoretically you can

achieve a narrow forward lobe (and hence a

very high DMF) using a very long Beverage.

To achieve the same 55° frontal lobe

beamwidth as from an 8-Circle antenna

(Section 1.30), you would need a 3-λ long

Beverage (~550 meters on 160 meters),

which poses two problems:

Not many have that much room for such long Beverages.

You run into space-diversity problems with this long an antenna, and

you will never reach the results predicted by modeling.

The only way to achieve a similarly narrow

forward beam and hence a very high RDF is

to use a broadside array of Beverages,

exactly the same as what we did with short

verticals in Section 1.11. You can obtain

excellent results with shorter Beverages (1

to 2 λ long) but these require a broadside

spacing of a little over λ/2. There is no free

lunch!

86

If the spacing is too wide, phase differences caused by space diversity will

negatively impact directivity.

87

This method requires a phasing line, and is therefore limited to one band.

There is a simpler method that permits multi-band use however.

88

End-fire phased Beverages at W3LPL

89

Broadside Phasing - A single 160-meter long Beverage (solid line);

two such Beverages in phase, side-by-side, spacing 40 meters

(dashed line) and 90-meter spacing (slightly over λ/2, dotted line).

Actual gain is irrelevant for receiving. Note very little little change

in pattern with 40-meter spacing other than 3 dB improvement in

gain (irrelevant for RX). With 90-meter spacing the frontal lobe is

much sharper.

End-Fire Phasing - Azimuth pattern on 1.83 MHz for a single 320-

meter Beverage (solid line); azimuth pattern for end-fire pair of

160-meter long Beverages, half the length (dashed line). Note that

while gain is greater with the long single beverage, the side and

rear lobes are very sharply reduced on the end-fire pair of shorter

Beverages.

90

Here is the W8JI’s description of the antenna: “Although it looks like a

large horizontal loop, this antenna actually is an array of two ground-

connection-independent end-fire staggered Beverages. The short end

wires form two single-wire feed lines at each end. One end-wire is series

terminated with exactly twice the resistance of a normal Beverage, while

the short wire at the opposite end is the feed-point.

Stagger and spacing determines feed and termination location, the offset

at the two ends being mirror images. A top view, looking down from

straight above the antenna is given in Fig 7-117.

Notice the feed point is offset towards the termination end (front) of the

antenna, and away from the null direction. This is typical for crossfire

phased arrays. Crossfire arrays respond away from the delay line

direction, exactly opposite conventional arrays. The termination is offset

the same amount, but moves towards the feed-point end of the antenna.

The feed-point terminals, being floated (push-pull), provide 180-degree

phasing between the two elements. The extra line length to the forward

(left and front) element provides the “Stagger” delay. Consider the actual

wire length of a “short side” called “X” (which is the same as Y1+Y2).

This length is the same on both ends of the antenna. The difference

between Y1 and Y2 must equal or be slightly less than stagger (S).To

determine the offset of feed point and load:

1. Measure length of the end-wire, length “X.”

2. Measure stagger in the end-fire direction, “S”

92

In the shack I have a control box with a knob surrounded by 8

LEDs. As the knob is rotated, a direction is selected, and the

corresponding LED is illuminated. I have two coax cables

running from the control box to three remote switchboxes in the

field (one switchbox feeds into another), and coax running to

the receive input on my rig. The switchboxes use relays to

switch both the center conductor and braid of the coax to

select the proper antenna i.e.: direction. Switching both

conductors is important - directivity can be lost if this is

not done. As a particular direction is chosen in the shack,

the appropriate coax is selected, and the proper control lines

energized so as to activate the required relays in the remote

switchboxes. Also, 24VDC is applied to the center conductor of

the coax that runs to the remote switchbox. More on that in a

moment.

I use a Kenwood TS-950SD on HF. It has two RCA plugs on back – RX Ant Out and RX Ant

In. Normally they are connected with a jumper, but they can also be used to connect a

separate RX antenna. I run a cable from both to the control box. Using the knob in the lower

right, I can connect the Beverage antennas, or several other RX antennas, including my

K9AY. I can also bypass the RX antennas, feeding the RX Ant Out into the RX Ant In. This

makes it easy to use other antennas when I go to higher bands.

The control box also takes a feed from the PTT line. When I transmit, a relay is keyed to

ground the input from the active Beverage. It might not actually be needed, but better safe

than sorry! There is also room for a pre-amp if I ever need one.

Why did I use 24VDC? I had lots of 24VDC relays and a 24 VDC power supply. It also

suffers from less I2R losses in the control lines and coax center conductor.

93

I use end-fire phased Beverages, using the crossfire phasing method. Because of the symmetry of the

feedpoint and termination resistor, I can get two directions from each pair. Each end of the array is fed

with 75 ohm cable. The antenna feedpoint at each end is connected to a terminating resistor through a

relay in a termination box. As described in the previous paragraph, when a particular direction is chosen,

the appropriate relays are activated to connect the coax from the proper antenna array to the coax running

from the control box. When this happens, 24 VDC is present at the termination box at the end of the

antenna. That voltage is used to activate a relay that switches the termination resistor out, and the

matching transformer in. Voila - the receiver is now connected to a pair of staggered phased Beverage

antennas! Of course, DC blocking capacitors are used in the appropriate locations in the termination

boxes and control box.

I found a use for empty anti-perspirant containers! Spray painted to protect from UV radiation, they

make great boxes for housing the terminating resistor, matching transformer, relay and associated

circuitry. The bottom is left open to allow moisture to drain. Although not apparent in the photo, the

termination boxes and coax switchboxes have LEDs to indicate when voltage is applied. This makes

troubleshooting easier.

Most of this was put up in the winter, with 1 meter of snow on the ground! It will be tidied up this fall.

94

I use 8-foot long 2x2 pieces of lumber, attached to shorter pieces driven into

the ground. Supports at the 4 corners are backed up with guy lines. Note the

coax coil RF choke. It needs several cores to increase the choking effect.

98

Unfortunately, the direction to Europe corresponds to the narrowest dimension of my property, about 320 feet. To improve performance

here, I am broadside phasing two pairs of echelon

phased Beverages i.e.: I have two sets of echelon

phased Beverage pairs, spaced at 300 feet, and fed

with equal lengths of coax into a signal combiner -

a total of 4 Beverages for these two directions. I

can also select just one pair of echelon phased

antennas if necessary.

Note that the length of a BOG will be less than that required for a Beverage

supported off the ground.

105

As the Beverage becomes longer, the angle of maximum gain monotonically

drops. When the length is 250 feet, the take-off angle is 66 degrees, a high

angle that is not usually useful for DX. Usual DX take-off angles range

between 10 and 30 degrees.

The first thing we notice about this graph is that it is not continuous, but steps

in units of 3 degrees. This is due to the simulation parameters, and the

selection of the angular spacing in the three-dimensional grid (EZNEC step

size). In other words, this graph is an approximation of the real data. Samples

are computed every 3 degrees. If you draw a smooth curve through this graph

you will have the actual elevation angle.

As we have seen before, the rate of change is faster at shorter antenna lengths.

The half-way point between 66 degrees and 18 degrees is 42 degrees. The

take-off angle reaches 42 degrees when the antenna is 570 feet long.

The elevation angle continues to drop as the Beverage becomes longer. At a

length of 8000 feet, the take-off angle is 6 degrees. It clearly cannot go below

0 degrees, and I do not know if it stops before 0 degrees.

106

I recorded the forward gain in two different ways. First, I measured the absolute maximum

forward gain. As the Beverage becomes longer, the maximum gain always increased. But, the

elevation (take-off) angle of maximum gain slowly drops, from 66 degrees to 18 degrees. In

dBi, the gain goes from -18.53 dBi to -8.31 dBi. Now we can see why a Beverage is usually

not used for transmitting. A half-wave dipole or quarter-wave vertical, even a poor one,

probably has a positive gain with respect to an isotropic radiator. This is a difference of

approximately 10 to 20 dB, which is a substantial difference.

I also measured the forward gain at a take-off angle of 21 degrees. I selected this angle

because it is a typical DX angle.

At the 250 foot length, the maximum gain is -18.53 dBi. At that same length, the gain at a 21

degree take-off angle is -22.27 dBi. As the Beverage gets longer, the two curves slowly

overlap, since the take-off angle of maximum gain drops to 21 degrees (and eventually goes

under 21 degrees). For all practical purposes, the two curves start to overlap at approximately

1200 feet. At 1800 feet, the maximum gain is -8.31 dBi, at an elevation of 18 degrees.

As the antenna becomes even longer, the gain continues to increase, and the elevation angle

continues to drop. The gain peaks at a length of 6000 feet. The gain at that length is -5.38 dBi,

and the take-off angle is 9 degrees. 9 degrees may be too low of an angle for typical DX

conditions. If we consider the gain at 21 degrees, the gain peaks at an antenna length of 2500

feet. The gain at that length, at 21 degrees take-off, is -7.82 dBi.

As we move from a 250 foot to 1800 foot Beverage, the gain at a typical DX take-off angle

increases by approximately 14 dB. The gain increase is monotonic. Gain always increases, it

never decreases, either at the maximum gain angle, or at 21 degrees.

As with the RDF, the rate of change of the gain slows down with increased antenna length. If

we select a length so that we achieve one-half of the overall gain difference, that length would

be 590 feet (-22.27 dBi + (14 dB/2) = -15.27 dBi).

107

T1 is a 7-turn transformer tapped at 5 turns (1.4:1 turns ratio, 2:1 impedance

ratio) step-down transformer. 73 material binocular cores are ideal for

100kHZ to 30MHz applications.

The magic T transformer is 5 to 10 turns of twisted pair wire through a 73

material binocular core. Configured as a center-tapped winding.

R1 is twice the expected load impedance. For 50- ohm systems use a 100 ohm

resistor.

109

The 3 dB beamwidth is a measure of the size of the forward lobe in the

azimuth plane. It is an angle, measured in degrees, and is the angle necessary

for the maximum gain to fall off by 3 dB in each direction

The beamwidth starts off quite wide, 215 degrees for a 250 foot long

Beverage. The beamwidth drops to 43.2 degrees for an 1800 foot Beverage.

It is interesting to note that unlike all previously presented measurements,

which changed monotonically, the reported beamwidth does actually back up a

few degrees from time to time. These are the very small spikes on the graph. I

do not know if this is a true characteristic of the antenna, or, some artifact of

modeling and simulation. I would tend to believe the latter.

It is also possible to measure the 3 dB beamwidth in the elevation plane.

Although I did not record that data, the trend is the same as for the azimuth

plane. The lobe narrows as the antenna becomes longer. If you combine the

azimuth and elevation beamwidths with the take-off angle, the picture that

emerges is of a rather large front lobe that both narrows and tips down as the

antenna becomes longer.

113

K9AY antenna at W8WWW

Aircraft detection system from the 1920s.

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